Grinding machine for grinding cylindrical workpieces

ABSTRACT

A grinding machine includes a rotating grinding wheel and workpiece moving apparatus operable to move a workpiece towards and away from the wheel. The apparatus is arranged to move a workpiece support arcuately about an axis to move a workpiece towards or away from the wheel. A motor turns the workpiece to present different parts of the workpiece to the wheel, and an orientation detector signals the orientation of the workpiece so that the operations of the workpiece moving apparatus, and the motor can be coordinated to cause a desired profile to be ground on the workpiece. The machine also comprises correcting apparatus operable in response to the operation of the workpiece moving apparatus to cause the output of the orientation detector to be altered to compensate for angular alterations in the point of contact of the workpiece with the grinding wheel caused by the arcuate movement of the workpiece support.

This invention is concerned with a grinding machine for use in grindingworkpieces to a desired shape. Such a grinding machine may be used, forexample, in grinding a cast cam blank to its final shape.

One known grinding machine comprises a grinding wheel having an abrasivecircumferential surface arranged to remove material from a workpiecewhen the grinding wheel is applied thereto, rotating means operable torotate the grinding wheel about a central axis thereof, a workpiecesupport arranged to support a workpiece so that the workpiece can turnabout an axis of the support, turning means operable to turn a workpieceon said support about the axis of the support to present different partsof the surface of the workpiece to the grinding wheel, orientationdetecting means operable to detect the orientation of the workpiece onthe support, moving means operable to bring about relative movementbetween the workpiece support and the grinding wheel, and control meansoperable to control the operations of the turning means and of themoving means in accordance with signals received from the orientationdetecting means so that the workpiece is ground to a desired shape.

The moving means may be arranged to move the workpiece support arcuatelyabout an axis of the machine to bring about said relative movement. Saidaxis of the machine is parallel to the axis of the grinding wheel andoff-set therefrom. This type of moving means is relatively simple andthe workpiece support can be positioned with considerable accuracy.However, the use of such a moving means can lead to inaccuracies in theshape ground because the arcuate movement of the workpiece supportalters the point of contact of the workpiece on the grinding wheelrelative to what it would be if the movement were linear. Thus, there isa discrepancy between the output of the orientation detecting means andthe position on the workpiece on which the grinding wheel is acting.This discrepancy can be taken into account by requiring the controlmeans to calculate a correction but either a complex and expensivecontrol means has to be provided or productivity is lost due to slowergrinding to allow time for the calculations to be made.

It is an object of the present invention to provide a grinding machinein which the aforementioned discrepancy is taken into account withoutrequiring calculations by the control means of the machine.

The invention provides a grinding machine comprising a grinding wheelhaving an abrasive circumferential surface arranged to remove materialfrom a workpiece when the grinding wheel is applied thereto, rotatingmeans operable to rotate the grinding wheel about a central axisthereof, a workpiece support arranged to support a workpiece so that theworkpiece can turn about an axis of the support, turning means operableto turn a workpiece on said support about the axis of the support topresent different parts of the surface of the workpiece to the grindingwheel, orientation detecting means operable to detect the orientation ofthe workpiece on the support, moving means operable to bring aboutrelative movement between the workpiece support and the grinding wheel,the moving means being arranged to move the workpiece support arcuatelyabout an axis of the machine to bring about said relative movement, andcontrol means operable to control the operations of the turning meansand of the moving means, in accordance with signals received from theorientation detecting means, so that the workpiece is ground to adesired shape, characterised in that the machine also comprisescorrecting means operable, in response to the operation of the movingmeans, to cause the output of the orientation detecting means to bealtered to compensate, at least substantially, for angular alterationsin the point of contact between the workpiece and the grinding wheelcaused by the arcuate movement of the workpiece support.

In a grinding machine in accordance with the invention, theaforementioned discrepancy is taken into account in the output oforientation detecting means which reaches the control means so that thecontrol means can operate in a simple manner.

In a machine in accordance with the invention, the correcting means maybe operable to move the orientation detecting means, or a portionthereof, to alter its output. For example, where the orientationdetecting means is a rotary device, such as an encoder, the correctingmeans may be operable to turn a portion of the device relative to theremainder thereof.

The machine, preferably, also comprises position detecting meansoperable to detect the position of the workpiece support relative to thegrinding-wheel. The position detecting means provides feedback to thecontrol means.

In a machine in accordance with the invention, the moving means maycomprise a motor which is operable to turn a cam which engages a rollermounted on an arm which carries the workpiece support and is pivotalabout said axis of the machine, the cam having a peripheral shape whichengages the roller, said shape being selected so that there is a linearrelationship between angular movements of the cam and movements of theworkpiece support towards or away from the grinding wheel.

There now follows a detailed description, to be read with reference tothe accompanying drawings, of a grinding machine which is illustrativeof the invention.

In the drawings:

FIG. 1 is a side elevational view of a portion of the illustrativegrinding machine; and

FIG. 2 is a view formed by opening out a section line passing throughfour axes of the machine A, B, C and D, these axes also being shown.

The illustrative grinding machine 10 comprises a grinding wheel 12 whichis mounted for rotation about a central axis (not shown) thereof. Thisaxis is on a fixed support on which is also mounted a motor (not shown)which provides rotating means operable to rotate the wheel 12 about thecentral axis thereof at a constant speed. The grinding wheel 12 (only aportion of the edge of which is indicated in FIG. 1) has an abrasivecircumferential edge which is arranged to remove material from aworkpiece when the rotating wheel 12 is applied thereto. The grindingwheel 12, its support and its motor are of conventional construction.

The machine 10 also comprises a workpiece support 14 arranged to supporta workpiece to be ground by being brought into contact with the edge ofthe wheel 12. In this case, the workpiece is a cast camshaft having camsprojecting therefrom and the machine 10 is set up to grind thecircumferential surfaces of the cams to their final shapes. The support14 carries a driver 16 which drives one end of the workpiece. Althoughnot shown in the drawings, the machine 10 also comprises a furthersupport which supports the other end of the workpiece. The support 14 ismounted on a spindle 17 which is mounted on bearings 18 for rotationabout an axis C which extends parallel to the central axis of the wheel12. Thus, the support 14 is arranged to support the workpiece so that itcan be rotated about an axis C of the support 14. Said further supportis also rotatable about the axis C. The machine 10 also comprises amotor 20 which provides turning means operable to turn the support 14,and hence a workpiece on the support 14, about the axis C to presentdifferent parts of the surface of the workpiece to the grinding wheel12. Specifically, an output shaft 21 of the motor 20 drives the spindle17 which in turn drives the support 14.

The bearings 18 and the motor 20 are mounted on an arm 22 of the machine10. The arm 22 is pivotally mounted on an axis B of the machine 10 bybeing fixed to a shaft 23. The shaft 23 is mounted on bearings 24mounted on a fixed frame 26 of the machine 10 for pivoting movementabout the axis B. The aforementioned further support for the workpieceis mounted on an arm (not shown) which is also fixed to the shaft 23 forpivotal movement about the axis B so that the opposite ends of theworkpiece move together.

The arm 22 also carries orientation detecting means of the machine 10which is operable to detect the orientation of the workpiece about theaxis C of the support 14. The orientation detecting means is provided byan angular encoder 28. The encoder 28 has an input shaft 30 on which ismounted a gear 32 which has 100 teeth around its circumference. The gear32 is meshed with a gear 34 which is mounted on the support 14 forrotation therewith about the axis C. The gear 34 also has 100 teetharound its circumference so that there is a one-to-one relationshipbetween rotation of the gear 34 and rotation of the gear 32. The encoder28, for reasons explained hereinafter, is mounted on bearings 29 on thearm 22 to turn about an axis D which is parallel to the axis C. Thecentre of the gear 32 is on the axis D. The encoder 28 produces anoutput of electrical pulses, the number of pulses being dependant on theamount by which the gear 32 turns relative to the encoder 28.

The machine 10 also comprises moving means operable to bring aboutrelative movement between the workpiece support 14 and the grindingwheel 12 in order to determine the amount of material removed from theworkpiece by the wheel 12. The moving means operates by moving the arm22 arcuately about the axis B, the extent of this movement beingindicated by the arc R in FIG. 1. The moving means comprises a motor 36mounted on the frame 26. The motor 36 has an output shaft 38 which turnsabout an axis A which is parallel to the axes B and C. The shaft 38carries a cam 40 which is mounted for turning movement about the axis Aon bearings 41 mounted on the frame 26. The cam 40 is positioned betweentwo rollers 42 which are mounted on the arm 22. The rollers 42 arearranged to engage an outer peripheral surface 44 of the cam 40 atpoints which engage the cam on opposite sides thereof. Operation of themotor 36 causes the cam 40 to turn about the axis A so that the rollers42 are moved by the surface 44. The movement of the rollers 42 causesthe arm 22 to pivot about the axis B thereby moving the axis C of theworkpiece support 14 along the arc R. An encoder 46 associated with themotor 36 provides position detecting means of the machine 10 operable todetect the orientation of the cam 40 and hence of the workpiece support14 relative to the wheel 12, since movement along the arc R moves theworkpiece support 14 towards or away from the central axis of the wheel12. The movement of the arm 22 is damped by a hydraulic damper 48 whichacts between the arm 22 and the frame 26 to which it is pivoted at 49.

The shape of the surface 44 of the cam 40 is arranged so that,throughout the range of movement, a unit angular turn of the cam 40results in a unit movement of the support 14 towards or away from thecentral axis of the wheel 12, ie there is a linear relationship betweenthe operation of the motor 36 and the resulting movement of the support14. This linear relationship is established to simplify operation ofcontrol means of the machine 10.

The control means of the machine 10 is provided by a programmableelectronic controller (not shown) operable to control the operations ofthe motors 20 and 36 in accordance with signals received from theencoders 28 and 46. The control of the motors 20 and 36 is to ensurethat the workpiece is ground to the desired shape. Thus, as theworkpiece is rotated by operation of the motor 20, its orientation aboutthe axis C is signalled to the control means by the encoder 28. Inresponse to the signals from the encoder 28, the control meansdetermines the appropriate orientation of the cam 40 for the portion ofthe workpiece's surface which is engaged by the wheel 12 and causes themotor 36 to operate to achieve that orientation. The encoder 46 providesfeedback that the correct orientation of the cam 40 has been achieved.

As the workpiece support 14 is moved arcuately, the effect of thismovement alters the point of contact between the workpiece and the wheel12 from what it would be were the support 14 moved linearly towards andaway from the central axis of the wheel 12. For this reason, the machine10 also comprises correcting means operable, in response to operation ofthe motor 36, to alter the output of the encoder 28 so that said outputis altered to compensate, at least substantially, for angularalterations in the point of contact between the workpiece and the wheel12. In other words, the correcting means alters the output of theencoder 28 so that the true orientation of the point of contact issignalled to the control means. Without the correcting means, thesignals would be due only to the operation of the motor 20 and for mostof the arc R could not represent the true orientation of the point ofcontact due to the non-linear nature of the movement of the support 14.

The correcting means comprises two gears 52 and 54. The gear 52 ismounted on the frame 26 and its centre is on the axis B about which thearm 22 pivots. The gear 52 is fixed against rotation and has 110 teetharound its circumference. The gear 54 has 98 teeth around itscircumference and is meshed with the gear 52. It should be noted thatthe gears 52 and 54 do not appear to be meshed in FIG. 2, because of theopened-out section but are, in fact, meshed. The gear 54 has its centreon the aforementioned axis D about which the encoder 28 can turn and isfixed to the encoder 28 to turn therewith in the bearings 29. The gear54 is not fixed to the shaft 30 of the encoder 28 but to the encoder 28itself.

In the operation of the motor 36, the arm 22 pivots about the axis B sothat the axes C and D are moved arcuately about the axis B. The arcuatemovement of the axis C moves the workpiece as described above. Thearcuate movement of the axis D causes the gear 54 to move around aportion of the circumference of the gear 52 so that the gear 54 isturned about the axis D and turns the encoder 28 about that axis. Theturning of the encoder 28 does not turn the shaft 30 which is heldagainst rotation with the encoder 28 by the meshing between the gears 32and 34. Thus, the correcting means comprises a fixed gear 52 mountedwith its centre on the axis B, and a turnable gear 54 mounted to bemoved arcuately about said axis B by the operation of the motor 36. Theturnable gear 54 is caused to turn by being moved arcuately asaforesaid, due to being meshed with the gear 52, and is fixed to theencoder 28. The turning of the encoder 28 about the axis D adds orsubtracts from the number of pulses which the encoder 28 sends to thecontrol means so that the output of the encoder 28 is altered tocompensate for the angular alterations in the point of contact. Theratio of 98 to 110 between the gears 54 and 52 is selected to suit theangle moved by the arm 22 plus the changing angle between axis C and thecentre of wheel 12.

What is claimed is:
 1. A grinding machine comprising a grinding wheelhaving an abrasive circumferential surface arranged to remove materialfrom a workpiece when the grinding wheel is applied thereto, rotatingmeans operable to rotate the grinding wheel about a central axisthereof, a workpiece support arranged to support a workpiece so that theworkpiece can turn about an axis of the support, turning means operableto turn a workpiece on said support about the axis of the support topresent different parts of the surface of the workpiece to the grindingwheel, orientation detecting means operable to detect the orientation ofthe workpiece on the support, moving means operable to bring aboutrelative movement between the workpiece support and the grinding wheel,the moving means being arranged to move the workpiece support arcuatelyabout an axis of the machine to bring about said relative movement, andcontrol means operable to control the operations of the turning meansand of the moving means, in accordance with signals received from theorientation detecting means, so that the workpiece is ground to adesired shape, wherein the machine also comprises correcting meansoperable, in response to the operation of the moving means, to cause theoutput of the orientation detecting means to be altered to compensate,at least substantially, for angular alterations in the point of contactbetween the workpiece and the grinding wheel caused by the arcuatemovement of the workpiece support.
 2. A machine according to claim 1,wherein the correcting means is operable to move the orientationdetecting means to alter its output.
 3. A machine according to claim 2,wherein the orientation detecting means comprises an encoder and thecorrecting means is operable to turn the encoder relative to its inputshaft.
 4. A machine according to claim 2, wherein the correcting meanscomprises a fixed gear mounted with its center on the axis about whichthe workpiece support moves, and a turnable gear mounted to be movedarcuately about said axis by the operation of the moving means, theturnable gear being meshed with the fixed gear so that it is caused toturn by being moved arcuately as aforesaid, the turnable gear beingdrivingly connected to the orientation detecting means.
 5. A machineaccording to claim 1, wherein the machine also comprises positiondetecting means operable to detect the position of the workpiece supportrelative to the grinding wheel.
 6. A machine according to claim 1,wherein the moving means comprises a motor which is operable to turn acam which engages a roller mounted to an arm which carries the workpiecesupport and is pivotal about said axis of the machine, the cam having aperipheral shape which engages the roller, said shape being selected sothat there is a linear relationship between angular movements of the camand movements of the workpiece support towards or away from the grindingwheel.